Spatially aligning two nominally flat surfaces so that they are parallel to one another and so that complementary features appearing on each of the surfaces are coincident is important in various applications such as surface mount technology, hybrid flip chip assembly, and solder reflow attachments with large numbers of interconnects. In order for the features on the two surfaces, such as solder, gold, indium and thick film bumps which are typically bonded thermally, ultrasonically, electrostatically, with epoxy or by RF techniques, to be properly mated when the surfaces are brought together, the surface features must be accurately aligned. Misalignment results in bad connections which cause failures and inoperability.
In order for the surfaces and the features on the surfaces to be properly aligned they must be aligned in five degrees of freedom. That is, they must be aligned laterally in the X and Y axes and angularly about the X, Y and Z axes. Then, the surfaces can be translated along the Z axis so that the surfaces and the features thereon can be mated properly.
Prior art alignment technology employs an imaging system for superimposing the images of two surfaces to be aligned. The superimposed images are then laterally aligned in the X and Y axes, as well as angularly about the Z axis. The remaining two degrees of freedom, i.e. rotation about the X and Y axes, are adjusted based on autocollimation signals returned from the two surfaces being aligned.
Autocollimation techniques use a specular return image signal from the surface and collimation is achieved when the returned image signal overlaps a target image. When aligning two surfaces, there are two returned images, one from each surface, that are caused to overlap by angular rotation of one surface about the system's X and Y axes. In order for autocollimation to work properly, the two surfaces that are being aligned must be smooth and fiat, like a plain parallel mirror. Textured and non-planar surfaces often fail to achieve a suitable return signal. The types of surfaces which require alignment, such as the applications discussed above, now contain more surface features and are more textured and thus are less reflective. Therefore, autocollimation is a poor choice for aligning these types of surfaces.
In addition, autocollimation techniques have a fixed accuracy which is in large part determined by the calibration of the particular alignment device. Alignment devices using autocollimation techniques include numerous adjustable optical elements that result in a complex and relatively unstable optical arrangement. Thus, autocollimation devices require, at a minimum, daily calibration. Moreover, as the size of the surfaces to be aligned increases, the accuracy of the autocollimation process decreases. Also, current alignment devices which incorporate autocollimation are not readily automated.
As the surface alignment technology is driven to smaller feature or bump sizes in larger array dimensions, the limitations of autocollimation has resulted in significant manufacturing yield reductions. Thus, a more versatile and effective alignment technique is needed.